Some of our HR Director's sent items disappeared this morning. He reported the problem and I went to watch.
I sent myself a test item. The item appeared in the Sent folder and in my inbox. About 30 seconds later, the item simply vanished out of the Sent folder (it remained in my inbox). It was a bit disconcerting.
A restart seems to have fixed the problem (without restoring vanished items) but he's a bit concerned that the problem may occur again without him knowing. Because I have no idea what caused the problem, I have to agree with him.